Conventionally, as paste materials for forming electrodes or circuits on surfaces of ceramic substrates, conductive pastes obtained by mixing metal particles such as Ag, Cu, Ni and W and glass particles (powders) into organic vehicles have been generally known. The conductive paste of this type is used by printing a predetermined pattern thereof on the substrate surface by screen printing or the like, and thereafter heating it at a temperature equivalent to or higher than the softening point of the glass component and equivalent to or lower than the melting point of the metal in a baking step. That is, in this paste, the glass component melted and softened by heating wets the ceramic substrate to cause bonding of the paste and the substrate, and the metal powders are sintered to one another, thereby forming a conductive part. Such glass particle-containing conductive pastes have been widely used in the field of electronics industry for the production of electronic components or circuit substrates from their easy handling. However, it cannot be denied that the bonding with the glass component is inferior in bonding strength to another bonding method. In particular, the glass component is difficult to wet the ceramic substrate such as an aluminum nitride substrate or a silicon nitride substrate, which is often used for the purpose of heat radiation of a high-power semiconductor, and it has been difficult to obtain the sufficient bonding strength. Therefore, in an application requiring high reliability, there is often used a thin film method by sputtering, a method of adhering a metal foil or a metal plate to a substrate using an active brazing metal, or the like. However, these metallizing methods are high in cost, and in the paste printing method requiring low cost, an improvement request for the bonding strength has still been strong.
Patent Document 1 discloses a metallizing method of strongly bonding an aluminum nitride substrate and a metallizing layer by adding a Ti compound (titanium hydride) as an active metal species into a conductive paste mainly composed of an Ag—Cu alloy. This document describes that baking is performed by heating in a non-oxidizing atmosphere, an inert atmosphere or a vacuum atmosphere, in order to prevent decomposition and oxidation of the Ag—Cu alloy and titanium hydride, and the baking is performed in the vacuum atmosphere in Examples.
In this method, the particles of the active metal or the compound thereof are used in combination with a suitable melting component (the Ag—Cu alloy in many cases), because the active metal alone is difficult to react with the substrate surface. Therefore, the paste easily flows during the baking to make pattern formation of an electrode or wiring difficult, and is often used for an application such as sticking of metal plates. Further, when baked in vacuum, the substrate and the metallizing layer are strongly bonded, and the pattern formation tends to be easily retained. However, the Ag—Cu alloy melted during the baking is contracted to cause turning-up of an end of the pattern or becoming small and round on the substrate, resulting in a tendency to deform the pattern.
This tendency is remarkable particularly in the baking in the nitrogen atmosphere (non-oxidizing atmosphere). In the baking in the nitrogen atmosphere, the pattern is completely turned up or deformed into a ball shape, and becomes a state where a part slightly in contact is barely bonded to the substrate. It is assumed that this is why the baking is performed in vacuum in Examples in this Document. Accordingly, the paste of this Document may strongly bond the fine pattern shape of the electrode, the circuit or the like to the substrate by the vacuum baking. However, the vacuum baking is a batch type, and moreover, requires a very long time for temperature rise or temperature drop. This is therefore low in productivity. When conveyer transportation type nitrogen baking is performed in order to improve the productivity, the metallizing layer is contracted. It is therefore difficult to retain the fine pattern shape of the electrode, the circuit or the like.
Patent Document 2 discloses a brazing filler material paste containing Ag—Cu alloy particles and titanium particles whose surfaces are coated with copper, as metal components. This Document describes that conventional titanium hydride is decomposed at about 400 to 500° C. to hydrogen and titanium, and that titanium having high reactivity reacts with oxygen or carbon of a decomposed gas of organic materials contained in the paste to hinder bonding to a ceramic member.
In contrast, Patent Document 1 discloses that titanium hydride is stable up to 700° C. However, in any case, the baking temperature is as high as about 850° C., and therefore, when exposed to the gas such as oxygen, carbon or nitrogen during the baking, it cannot sufficiently function. Thus, the baking has usually been performed in the above-described vacuum containing no gas.